The invention relates to a press pad made of an asbestos-free material for multi-story high-pressure presses for the production of high-pressure laminates.
Such high-pressure presses are employed to produce decorative or industrial high-pressure laminates. These presses generally operate in a pressure range between 85 and 100 kP/cm.sup.2 and at temperatures between about 130 and 160.degree. C. The pressing time is about 20 to 120 minutes.
In such high-pressure presses, press pads serve the purpose of transferring the pressure over the full area of the laminate, and must be able to withstand the stated high pressures and temperatures.
Conventionally, kraft paper is employed as the press pad for high-pressure presses in a desired number of kraft paper layers. However, this has the disadvantages that heat does not pass very well through the press pad, and that the production cycles are relatively long.
Occasionally rubber pads are also employed.
Press pads are also available for low pressure presses, those operating in a pressure range up to about 35 kP/cm.sup.2 ; these pads are composed of a textile fabric made of a yarn of aromatic polyamide and containing copper filaments in a proportion between about 80 to 95 weight percent with respect to the total weight of the press pad. These prior art press pads cannot be employed in high-pressure presses because the high pressure in the press would cause the metal filaments to cut through the yarn.
There are also press pads made of a textile material having a metal content of about 50 to 65%, but these pads are employed in multi-story presses for coating particle boards. Compared to multi-story presses for the production of high-pressure laminates, these presses operate with lower specific, operating pressures, namely up to about 65 kP/cm.sup.2, and with higher temperatures, namely at about 160 to 200.degree. C. Pressing times ar about 20 to 60 minutes.
In order to clarify the problems on which the invention is based, the requirements for a press pad for high-pressure presses will now be described in greater detail.
A press pad must be able to be deformed repeatedly under the influence of pressure, and over a longer period of time, while transferring pressure over the full area of the material being pressed (the laminate). Moreover, the press pad must be able to recover from the repeated compressions to such an extent that it is able to be sufficiently deformed during the next pressing process. These characteristics are significant factors for the service life of a press pad.
In high-pressure presses for the production of laminates, so-called decorative or industrial high-pressure laminates (HPL), kraft papers are employed at the present almost exclusively as press pads in a required number of layers. This has the drawback that the layers of paper must be cut and stacked at high costs of time and personnel. Moreover, the service life of these paper pads is short, the paper generally becoming brittle and worn after about 20 uses. It can then no longer be employed and must be discarded.
A press pad must also have good thermal conductivity independent of the thickness of the pad.
The thermal conductivity of the presently employed kraft papers is very low. In order to obtain a good padding effect, a plurality of kraft paper layers are required. But with every additional layer of paper, the quantity of heat transferred decreases. In practice this means that the heat loss must be compensated for by extending the time over which there is heating of the material being pressed or by increasing the temperature. High-pressure presses for the production of laminates (high-pressure laminates) generally operate in a pressure range between 85 and 100 kP/cm.sup.2 and at temperatures between about 130 and 160.degree. C.
Press pads must be able to withstand these high pressures and temperatures.